By the hydraulic-composition bonded magnet is meant a material in which particles of a magnetic powder are uniformly held in a matured/cured hydraulic composition as a bonding agent.
There was proposed an R—Fe—B based permanent magnet of high magnetic properties, which utilizes Nd and Pr, light rare earth elements abundant as resources, and B and Fe as main components and which has an excellent corrosion resistant film (Japanese Unexamined Patent Publication No. Hei-10-154611). This R—Fe—B based permanent magnet is made by putting a film layer having a Si—Na—O based glassy material of a specified film thickness and a fine crystalline material on the surface of an R—Fe—B based permanent magnet body having a tetragonal system as the main phase. In this conventional art, a rare earth-based bonded magnet is produced by using water glass (sodium silicate) as a binder.
However, this rare earth-based bonded magnet does not exhibit sufficiently improved moldability, heat resistance, corrosion resistance and strength, and thus was subjected to the following improvements in terms of moldability, heat resistance, corrosion resistance and strength. These were attributed to a basic problem inherent to conventional rare earth magnets. That is, they are likely to rust because of being made from active metal materials, which leads to the deterioration of magnetic properties. In order to solve this rust problem and improve the properties, a surface of a hard magnetic powder has been subjected to conversion treatment such as phosphate treatment or chromate treatment to form an oxidation-resistant conversion film (Japanese Unexamined Patent Publication No. Hei-1-14902), subjected to vapor deposition of Zn or Al, or electroless Ni plating (Japanese Unexamined Patent Publication No. Sho-64-15301), or an inhibitor such as sodium sulfite has been added to a resin binder (Japanese Unexamined Patent Publication No. Hei-1-147806). However, such surface treatment primarily focuses attention on the improvement of corrosion resistance, and does not pay attention to properties (adhesion properties, strength) resulting from the combination with a resin binder, the largest characteristic of a bonded magnet, and therefore problems with moldability, strength and magnetic properties still remain.
In addition, further with these problems, a method of making a silicon dioxide protective coat (hereinafter, referred to as “SiO2 coat”) on the particle surface has been investigated. However, it is not easy to form a homogeneous, dense, strong SiO2 coat on the surface of a magnetic powder having complicated shape and surface structure and having a particle size of the order of μm. Japanese Unexamined Patent Publication Nos. Sho-62-152107 and Hei-8-111306 also propose a method of forming a SiO2 coat or a silicate protective film on the particle surface. However, it is technically impossible to coat the particle surface with a 100% complete film.
Furthermore, the method disclosed in Japanese Unexamined Patent Publication No. Sho-62-152107 utilizes a reaction active silyl isocyanate, but this method is difficult to grow uniform nuclei and is likely to produce an uneven film. It is impossible to make a stiff coat by uneven physical adsorption on a magnetic powder only using a silicate. On the other hand, Japanese Unexamined Patent Publication No. Hei-8-111306 discloses a method of forming a SiO2 coat on a magnetic powder surface by using ethyl silicate by means of a sol-gel process, or a plasma chemical vapor deposition. However, the film thickness is as thick as 0.1 to 2.0 μm as is produced by the conventional sol-gel reaction, and so the film is not homogeneous, dense and strong.
Additionally, for bonded magnets using a Fe—Nd—B based alloy powder, a variety of methods have been studied that involve producing a bonded magnet by coating a magnetic powder with a resin as well as performing oxidation-resistant and corrosion resistant treatment by the oxidation film process. For example, Japanese Unexamined Patent Publication No. Sho-51-38641 discloses a method, which involves using a thermosetting resin (epoxy resin), and Japanese Unexamined Patent Publication No. Sho-50-104254 discloses a process, which involves using a thermoplastic resin (nylon). However, for the method which utilizes an epoxy resin, the mold flowability during compression molding is poor, heat treatment hardening (hereinafter referred to cure) after molding is required, the contraction coefficient is large (2 to 5%), and the molded body produced is not practically usable under a high temperature (150° C. or higher) environment, and further the molded body must be subjected to high melting point resin coating or surface treatment such as plating in order to improve the corrosion resistance, yet it is not sufficient for preventing the generation of rust. Moreover, an injection molding magnet using a thermoplastic resin such as a nylon resin is proposed as well, but the magnet causes a problem relating to corrosion resistance that it is rusted by water absorption even if the resin is uniformly coated on the powder surface, inasmuch as the surface treatment of the powder is not performed, or even though it is done, the method is not optimal.
In addition, for a heat resistant, consideration is conventionally made to the heat resistance only when the magnets are used. For example, with general flow solder or reflow solder, treatment is performed at a high temperature of 230 to 270° C. Accordingly, when molding is carried out using a nylon resin or epoxy resin, the resin cannot hold its shape at such a high temperature resulting in deformation, which in turn causes the problem of adversely affecting the function as a magnet material.
In order to solve the above problems, Japanese Unexamined Patent Publication Nos. Hei-2-22802 and Hei-2-281712 disclose a method that involves coating a rare earth magnetic powder with a super-engineering resin such as a polyether ketone or polysulfide ketone and subsequently compression molding, injection molding or performing extrusion. However, the method which utilizes a super-engineering resin cannot uniformly coat a powder either because of poor wettability between the powder surface and the resin, and also has difficulty in molding. As a result, this method has not been put into a practical use. Additionally, of super-engineering resins, even a material using a polyphenylene sulfide resin (PPS) of relatively easily being compounded generates sulfur dioxide gas during kneading or during molding by heating. Also, compounding with a ferromagnetic powder contained in an amount exceeding about 70% produces large adverse effects on magnetic properties and physical properties of the magnetic powder due to the necessity of a very high temperature and high shear, leading to difficulty of highly filling.
As discussed above, it is impossible to completely suppress the generation of rust by the conventional method that involves applying oxidation resistant treatment and corrosion resistant treatment on the surface of a rare earth element-based hard magnetic powder, or hardening with a resin to produce a bonded magnet. Accordingly, it is impossible to produce a bonded magnet material that has high magnetic properties as well as heat resistance and corrosion resistance by the conventional method.